Irish Water is planning a €900m investment in wastewater treatment in Dublin.

A €400m upgrade of the Ringsend plant, and development of a new €500m wastewater facility at Clonshaugh, in north Dublin, will improve water quality and increase capacity across the network by 50pc.

This morning Housing Minister Eoghan Murphy will mark the start of work on the Ringsend upgrade, with the first phase costing €80m.

Irish Water managing director Jerry Grant has confirmed planning permission will be sought in the coming months to develop a new €500m wastewater plant at Clonshaugh, which will treat wastewater from the west and north of the capital.

In all, investment of €900m is planned out to 2025.

Wastewater from 40pc of the State's population is processed in Ringsend, which was updated in 2005 and is designed to treat effluent from 1.64 million people.

But the plant is "significantly overloaded", and is treating wastewater from the equivalent of 1.9 million people.

There are concerns that nitrogen and phosphorous are not being adequately removed from the wastewater before it is discharged into the Liffey estuary, which is having an impact on water quality.

A previously approved upgrade included a 9km pipe into Dublin Bay, where processed wastewater would be discharged, but Irish Water intends scrapping this plan.

This will save between €300m and €700m, Mr Grant said, due to the risks that are involved in developing a sub-sea pipeline.

Instead, new technology called aerobic granular sludge will be deployed, which is already in use at smaller plants in Clonakilty, Carrigaline and Carrigtwohill, in Cork, and which the utility says successfully removes phosphorous and nitrogen.

Testing of the technology at Ringsend has been undertaken since April 2015, and the first stage of the upgrade involves developing six new tanks where it will be deployed to treat wastewater. This phase of the works will be launched by Mr Murphy today.

Some €70m has already been spent on the works, and, over the coming years, the existing 24 tanks will be upgraded and other works completed, resulting in an overall upgrade bill for Ringsend of €400m.

"With taking out the tunnel, it's a tremendous opportunity to save costs, and take out a huge body of risk and end up with a much more sustainable plant," Mr Grant said. "This is a much more secure, long-term option.

"This is a priority project for Irish Water and is vital for future social and economic development in Dublin, for public health and for improving the waters and ecology of Dublin Bay.

"It will take us in total, allowing for planning, up to about 2023, but we expect to be fully compliant for the nitrogen and phosphorous problem by 2021."

Planning permission will be sought for the complete upgrade in the coming months, and a separate application will be made for the Greater Dublin Drainage Scheme, which includes a new wastewater treatment plant at Clonshaugh, and which is expected to be operational by 2025.

Mr Grant said that the cost of developing a new water supply for the capital, where water is sourced from the Shannon before being pumped to Dublin, is expected to be €1.5bn.